The present invention relates to a method of reducing the level of cyclic dimethyl polysiloxane and low molecular weight dimethyl polysiloxane (referred to as "dimethyl polysiloxane" in the following description) which, for example, remain in a silicone tube, color cap or the like (referred to as "silicone rubber product" in the following description) which generate a silicon gas. More particularly, the present invention relates to a new method of reducing dimethyl polysiloxane to an ultra-micro level, which enables the level of dimethyl polysiloxane to be reduced to an ultra-micro level in a very short time and at a high efficiency by performing an ultrasonic treatment while immersing the silicone rubber product in an organic solvent while under agitation.
Silicone rubber products have been utilized in many fields for structures which undergo extremes of temperature, such as automobiles, aircraft, electric appliances or the like. Especially, although electronic circuits are small sized and integrated as well as improved in reliability, the relays, switches or the like used with these circuits are also small sized to employ a low force of contact. Contact hindrance of metal contacts of the relay and switch are almost always due to bad contact. The main factors causing bad contacts are dust and harmful gas (for example, corrosive gas and organic gas). The harmful gas is classified into corrosive gases present in the atmosphere, such as hydrogen sulfide, sulfur dioxide, ammonia gas or the like and organic gases which are generated from precursor materials such as silicone compounds, adhesives or the like (e.g., construction materials of instruments). It has been already reported that the gas generated particularly from silicone rubber products has a remarkably undesirable influence on metallic contacts (see, for example, "Quality Control", Vol. 27, No. 11, pages 70-76, 1976; "Influence of Gas Generated from Si Rubber to Metallic Contacts" by Takeshi Aoki (Nippon Telegraph and Telephone Public Corporation) and Hiroshi Oikawa (Yokosuka Electric Communication Institute) in "EMC - 79-42", pages 23-29, 1979).
Silicone rubber is generally synthesized by using cyclic dimethyl polysiloxanes comprised primarily of polysiloxanes of the formula [(CH.sub.3).sub.2 SiO].sub.4-6 in order to form via ring opening polymerization a silicone rubber high polymer. However, other dimethyl siloxanes defined by the formula [(CH.sub.3).sub.2 SiO].sub.3-25 are present in the reaction mixture and may not enter into the polymerization process.
It has been already made clear from the aforementioned documents that the source generating the silicone gas is a cyclic dimethyl polysiloxane and a low molecular weight dimethyl polysiloxane remaining in the silicone rubber product as unreacted residual components.
Such unreacted dimethyl polysiloxane is generally treated at a temperature of 200.degree. C. for 4-24 hours by means of a treatment method which includes a second vulcanizing stage subsequent to a normal vulcanizing (forming) step being performed, for example, by means of a hot air circulating dryer. Also, in a treatment method using a vacuum dryer, the amount of dimethyl polysiloxane present may be reduced by treatment at a temperature of 200.degree. C. for about 1-4 hours.
However, large quantities of dimethyl polysiloxane remain subsequent to either of the above treatment methods using the hot air circulating drying system or the treatment method using the vacuum drying system, regardless of the length of time of the treatment performed. Also, the treatment time is long, and the products contained in the treatment reservoir during the treatment contact each other as well as the treatment reservoir. Further, the treatment varies because it is very difficult to maintain a uniform temperature distribution in the dryer. Problems thus result with regard to the quality of the product produced as well as productivity.
The inventors have diligently attempted to provide a method to reduce the amount of dimethyl polysiloxane remaining in the silicone rubber product which product exhibits excellent heat resistance, cold resistance, ozone resistance, electric insulation or the like, and as a result of this, have succeeded in developing a novel method of reducing the amount of dimethyl polysiloxane remaining in the silicone rubber product to an ultra-micro level over a very short time and with high efficiency.